In certain manufacturing operations, particularly those for fabricating disc memory thin-film magnetic heads in situ on the air bearing slider to be carried by the head arm, it is desirable to machine the flying surface until a precisely located line on another surface intersecting the flying surface becomes the line of intersection of the two surfaces. In the thin-film head example, the head is carried on an end face of the slider which is approximately perpendicular to the flying surface, and the line is positioned to specify very accurately the thin-film head's throat height, that is the dimension of the flux gap normal to the transducing surface. (The transducing surface, of course, is nearly parallel during disc memory operation, to the medium surface.) Accuracy in throat height to within a few tens of microinches is desirable to insure optimum electronic and magnetic characteristics. Machining the flying surface until it coincides with the desired line of intersection then automatically sets throat height to the accuracy with which the line of intersection was set.
Controlling this dimension during fabrication has always been a difficult problem because of the extremely small dimensions and tolerances involved. Simply using the top of the slider prism as a reference surface for controlling throat height was satisfactory when grinding ferrite heads, see U.S. Pat. No. 3,982,318. But tolerance and dimensions are much larger in ferrite head technology.
Respecting thin film heads, recent innovations allowing accurate control of throat height involves the use of so called lapping guides or machining sensors, e.g., as disclosed in IBM Technical Disclosure Bulletin (TDB) Vol. 23, No. 6, November 1980, p. 2550. These guides or sensors are deposited conducting materials placed on the surface carrying the thin-film head. Two types of sensors are in general use. So-called discrete sensors simply have their electrical continuity broken at some point during machining and hence, provide an indication of machining progress at only a single instant. Analog sensors have an area of resistive material which is slowly removed by machining and hence provide a continuous indication until continuity is broken. With respect to discrete sensors, typically several at different heights are employed. The continuity of each is successively broken by the machining process, thereby providing a series of indications of precisely how much more machining must yet occur to reach the desired final position line. At the limits of or within the desired throat height range, a last sensor's conductive path will be opened signaling that the machining process should stop.
The use of these machining sensors drastically improves the accuracy with which the edge can be positioned relative to the feature. However, when dealing with thin-film magnetic heads, one cannot form conventional machining sensors with the same step which defines the throat of the gap. This is because the throat is formed by the deposition of an insulating layer, whereas the machining sensors are conductive patterns and hence are deposited in the steps creating the magnetic legs of the head. It is a known difficulty that successive layers of material deposited by the use of photo-optic masks and forming a composite thin-film structure cannot be registered with respect to each other with perfect accuracy. That is, the masks or patterns which define each of the features of successive layers such as the bottom leg, the throat and the top leg, cannot be placed in precise alignment with the patterns created by previous masking steps during typical manufacturing operations. Therefore, the throat height of a typical thin-film head cannot be controlled to an accuracy greater than the registration between the throat insulation-forming pattern and the magnetic leg/machining sensor-forming pattern. Experience shows that this inherent inaccuracy results in a substantial percentage of head gaps which have throat heights outside of the required tolerances. Worse still, even though the throat height-defining step occurs intermediately in the process, one cannot easily tell whether or not the head is good until the manufacturing process is complete, making the relatively high number of reject heads an expensive flaw in these previous systems.
The problem of aligning machining sensors with a feature formed of insulating material such as the throat defining layer of a thin-film head is present for both discrete and analog sensors. In a current manufacturing process, analog sensors are used to indicate the progress of machining of a workpiece carrying several thin-film heads. The machining step sets the throat heights for all the thin film heads simultaneously. An analog sensor is interposed between each pair of heads. It is necessary that the position of each analog sensor vis-a-vis its adjacent heads be known very accurately so that machining can be halted when the throat heights of as many heads as possible are within the desired tolerances. (Due to various inaccuracies in the process, it is possible that not all throat heights can be reduced to a value within the tolerance range at the same time.) Such a process is described in U.S. patent application No. 06,430,195, entitled Workpiece Carrier, and having the same filing date and assignee as this application.
IBM TDB Vol. 18, No. 1, June 1975, p. 227, recognizes the difficulty in aligning features of different deposition layers and apparently teaches depositing the lapping control layer with the same step which forms the "registration of the insulating layer forming the gap or covering the gap layer." How an insulating layer can be registered in the same step with depositing the lapping control layer isn't explained.
IBM TDB Vol. 23, No. 2, July 1980, p. 776, teaches a method of calibrating an analog lapping guide or machining sensor to compensate for variations in bulk resistivity and film thickness. This method is not involved with determining position of the analog sensor relative to a feature of an insulating layer.